Method of rolling stainless steel and product thereof



Patented May 24, 1938 UNITED STATES PATENT OFFICE METHOD OF ROLLINGSTAINLESS STEEL AND PRODUCT THEREOF William B. Arness, Baltimore, Md.,assignor to Rustless Iron and Steel Corporation, Baltimore, Md., a.corporation of Delaware No Drawing. Application January 21, 1937, SerialNo. 121,668

19 Claims.

strong, durable, heat-resistant and corrosionresistant; which possesshigh tensile strength, and high impact value; which lend themselves tohot and cold working, hardening, polishing, and the like; and which arehighly resistant to decarburization and grain growth under the manyconditions offabrication and use. v

The invention, accordingly consists in the combination of elements,composition of ingredients,

used corrosion-resisting ferrous alloys, either the ferritic rustlessalloys of the iron-chromium group or the austenitic iron-chromium-nickelgroup, many highly desirable characteristics are achieved. These alloysare durable, strong, and tough; they may be worked either cold or hotfrom strip, sheet or bar stock to give products or articles of desiredsize and shape; they are resistant to the corrosive efiects ofatmospheric conditions as well as to many acids, alkalies and salts,and, finally, are resistant to the efiects of high temperatures;resisting discoloration and scaling while retaining their physicalcharacteristics, strength, toughness and durability.

ln heretofore known and/or used stainless iron or steel products of theclass indicated products of no one composition, however, are equallypossessed of all of the beneficial charactertistics outlined above;Thus, for example, on the one hand a product having a chromium contentnear the lower limit for commercial rust resisting alloys (about 12%)lends itself to both hot and 50 cold working, may be heat-treated togive a fair range of hardness, strength, and impact resistance, and isnot particularly susceptible to decarburization and grain growth. Thealloy, however, is not qualified for resistance to severely corrosiveconditions, for extreme high temperature duty, and especially for hightemperatures in corrosive media; which characterizes the ironchromiumalloys of higher chromium contents.

Similarly, on the other hand an iron-chromium, product having a chromiumcontent near the upper commercial limit for rustless ferrous alloys(about 2'7 to 30%) although highly resistant to corrosion, and resistantto discoloration and scaling at high temperatures, suffers inworkability and ductility. In addition, such a product in itsmanufacture tends toward decarburization and surface grain growth, witha resultant roughening of the surface of the metal and the formation ofminute checks and pits. It does not lend itself to hardening byheat-treatment, and such physical characteristics as tensile strengthand impact resistance are limited. It is difficult to machine and mustbecarefully handled in both hot and cold forming operations to ensure asatisfactory product. It possesses a somewhat coarse grain tendingtoward growth, under contlnued use at high temperature thus causingbrittleness, low impact, and low fatigue resistance.

Likewise, iron-chromium-nickel stainless steel sheet, strip, wire, andthe like, although ductile and workable (both hot and cold), readilyweldable and heat and corrosion resistant, are not hardenable byheat-treatment and are subject to decarburization and surface graingrowth in manufacture as well as grain growth in use at hightemperature.

One of the objects of my invention is the production, in an inexpensiveand eficient manner, of corrosion resistant plate, sheet, strip, wire,and the like of uniform fine grained texture which are less subject todecarburization and grain growth, brittleness and fatigue than in thecase of heretofore known products; which are of improved workabilityover a wider range of chromium contents than for the products describedabove; and which, for the ferritic alloys, lend themselves to increasedmaximum hardening by heat-treatment and hardening by heattreatmentthroughout a wider range of chromium J are largely determinant of thephysical charsmall amounts of nitrogen giving a corrosion-resistingferrous alloy of fine even grain structure which is especially resistantto decarbui'ization and grain growth in subsequent working of ingots,blooms, slabs and billets into plate, sheet, strip, bars, rods, wire andthe like as indicated hereinafter. The evenness of grain is useful inminimizing directional properties in plate, sheet, strip, bars, rods andwire where the total extent of reduction is over about 50%. Italso isbeneilcial in cold-rolled sheet and strip even where the amount of thetotal cold reduction is in excess of about 35%, the cold-working itselfto some extent aiding in the elimination of directional effects.

Preferably the proportions of ingredients added are such as to give astainless steel product analyzing approximately 10% to 30% chromium,

.06% to .30% carbon, and over .06% but under 20% nitrogen for. theferrltic iron-chromium metal (or chromium up to 30%, nickel up to 30%,carbon below .30%, and nitrogen greater than .06% but less than 20% forthe austenitic ironchromium-nickel metal). The particular quantity ofthe alloying metals together with the precise amount of carbon andnitrogen present acteristics of the products as will appear more fullyhereinafter. A nitrogen content in excess of .06% and preferably inexcess of .07% is necessary to impart the desirable characteristics tothe products while a nitrogen content of '.20%

or over is likely to result in unsoundness and other gas defects.Ordinarily best results are achieved where the nitrogen content isgreater than about .07% but less than about .13%, say approximately.09%.

The hot working characteristics of the metal are not adversely affectedby the presence of nitrogen, the alloy lending itself to forging, up-

. setting, swaging and like hot operations, while the cold-workingcharacteristics such as beading, spinning and deep-drawing are somewhatimproved for all chromium (or chromium and nickel) analyses; theimprovement in deep-drawing characteristics being particularly improvedfor a chromium content of 16% to 20%.

In fact, as a result of the resistance to decarburization advantage maybe taken of higher working temperatures in processing ingots to billets,sheet, strip and bars, either by hot-worka ing or by a combination ofhot and cold-working operations, without resultant decarburization,surface grain growth, and generally coarse structure. The absence ofdecarburization and surface grain growth aids materially in producing asmooth. bright surface which lends itself to easier polishing, therebyeffecting important economies in this costly operation.

The properties conveyed by the nitrogen content are in some respectssimilar to the well a known properties resulting from the use of a smallincreased amount of carbon without, however, the attendant undesirableeifects of carbon on corrosion resistance and without decarburizationand surface grain growth. In-addition, it seems that the presence ofnitrogen increases the solid solubility of carbon in the metal attemperatures above the critical point with a resultant more uniformdispersion of carbides in the metal, as martensite, upon cooling to roomtemperature.

As indicated above, the products are of a fine, even grain structure. Aneven, or uniform, grain structure is one which is less susceptible tothe anaooa segregation of carbides. An even grain structure isparticularly beneficial in such hot-worked and cold-worked products asplate, sheet, strip, bars and rods because there is a minimum ofresultant directional properties. The freedom of these products from theusual striated or fibrous character found in heretofore known productsis a very important characteristic. Especially is this true where theproducts such as sheet and strip are subjected to severe coldformingoperations as in deep-drawing. Products of a fibrous or striatedcharacter result in nonuniform forming and frequently in tearing andsplitting of the metal. a

In addition to the even grain structure, the products are especiallyresistant to decarburization and surface grain growth, this latterfeature becoming increasingly important in products of the higherchromium contents and especially those of a chromium content between 16%and 20%.

Flowing from these characteristics are many practical advantages.Objectionable general grain growth as well as decarburization andsurface grain growth of the products of the higher chromium contents areappreciably decreased. The products are more ductile, more workable andfor the ferritic iron-chromium products, lend themselves to hardening byheat-treatment over a wider range of chromium content than in heretoforeknown rustless products or manufactures of the kind indicated. Coldforming operations, such as blanking, stamping, and especiallydeep-drawing, are considerably improved while hot forming operations maybe successfully carried out over a wider range of temperatures.Likewise, the tensile strength of the products may be appreciablyincreased throughout a broader chromium range. So also, may the impactresistance of the lower chromium products be increased.

Furthermore, as a result of the fine grain structure and increasedresistance to grain growth mentioned above, the products more readilylend themselves to welding and, in addition, give a weld of finer grainstructure and hence one that is more reliable than in heretofore knownproducts of the class indicated.

In addition to the many highly beneficial characteristics outlinedabove, the heat-resisting characteristics of the products areconsiderably improved over heretofore known and/or used products of theclass indicated since resistance to objectionable grain growth isbettered as mor particularly pointed out above.

Other characteristics of stainless steel products of the classindicated, such as resistance to corrosive eflects of acid, alkaline andsalt solutions are fully retained and, as a result of the finer grainstructure, are substantially improved.

Thus it will be seen that there has been provided in this inventionstainless steel or rustless iron and steel products in which the variousobjects hereinbefore noted together with many thoroughly practicaladvantages are successfully achieved. It will be seen that the physicalcharacteristics of these products are greatly improved; that theseimproved characteristics permit many savings in manufacture and useheretofore unrealized; and that the useful field of application of theseproducts-is appreciably broadened.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiment hereinbefore set forth, it is tobe understood that all matter described which includes, heating orre-heatlng ingots,

blooms; slabs or billets comprising chromium per cent to 30 per cent,nitrogen in an amount less than .20 per cent but over .06 per cent, andthe balance iron and then hot-rolling said prodnets to form sheet orstrip free from decarburization and surface grain growth effects byvirtue of said nitrogen content.

2. In the production of stainless steel sheet or strip substantiallyfree of surface defects, the art which includes, heating or re-heatingingots, blooms, slabs or billets comprising chromium 10 per cent to 30per cent, nitrogen in'an amount less than .13 per cent but over .07 percent, and

the balance iron, then hot-rolling said products to form sheet or stripfree from decarburlzation and surface grain growth effects by virtue ofsaid nitrogen content, and then cold-rolling this hotrolled sheet orstrip.

"which includes, heating or re-heating ingots,

blooms, slabs or billets comprising 0 romium 10 per cent to 30 per cent,nitrogen in an amount less than .20 per cent but over .06 per cent, andthe balance iron, then hot-rolling said products to form sheet or stripfree from decarburization and surface grain growth effects by virtue ofsaid nitrogen content, cold-rolling this hot-rolled sheet or strip, andthen polishing this cold-rolled sheet or strip'thereby obtaining amirror-like surface.

4. In the production of stainless steel rods or wire substantially freeof surface defects, the art which includes, re-heating a billetcomprising chromium 10 per cent to 30 per cent, nitrogen in an amountless than .13 per cent but greater than .07 per cent, ,and the balanceiron, and then rolling and drawing said product forming rods or wirefree from decarburization and surface grain growth effects by virtue ofsaid nitrogen content.

5. In the production of stainless steel rods or Wire substantially freeof surface defects, the art which includes, re-heating a billetcomprising chromium 10 per cent to 30 per cent, nitrogen in anamount'less than .20 per cent but greater than .00 per cent, and thebalance iron, then rolling and drawing said product forming rods or wirefree from. decarburization and surface grain growth effects by virtueofsaid nitrogen content, and then polishing this rod or wire to achievea desired lustrous surface.

6. In the production of stainless steel sheet or strip substantiallyfree of surface defects, the art which includes, heating or re-heatingingots, blooms, slabs or billets comprising chromium 16 per cent to 20per cent, carbon .06 per cent to. .30 per cent, nitrogen in an amountgreater than .06 per cent but less than .20 per cent with the balanceprincipally iron, and then hot-rolling said products forming therebysheet or strip free from decarburization and resultant surface graingrowth and minute pitting.

'7. In the production of stainless steel sheet or strip substantiallyfree of surface defects, the art which includes, heating or re-heatin'gingots, blooms, slabs or billets comprising chromium 16 per cent to 20per cent, carbon .06 per cent to .30 per cent, nitrogen in an amountgreater than .06 per cent but less than .20 per cent with the balanceprincipally iron, then hot-rolling said prod-' ucts forming therebysheet or strip free from decarburization and resultant surface graingrowth and minute pitting, and cold-rolling this sheet or strip toachieve a desired bright shiny surface.

8. In the production of stainless steel sheet or strip substantiallyfree of surface defects, the art which includes, re-heating and thenhot-rolling ingots, blooms, slabs or billets comprising chromium 16 percent to 20 per cent, carbon .06 per cent to .30 per cent, nitrogen in anamount greater than .07 per cent but less than .13 per cent with thebalance principally iron forming thereby sheet or strip free fromdecarburization and resultant surface grain growth and minute pitting,cold-rolling this hot-rolled sheet or strip, and then polishing thecold-rolled product to achieve a mirror-like surface.

9. In the production of stainless steel sheet or strip for use in thefabrication of pressed, bent, stamped or deepdrawn articles, the artwhich includes, re-heating and then hot-rolling ingots,

blooms, slabs o'r billets comprising chromium 10 per cent to 30 percent, nitrogen greater than .06 per cent but less than .20 per cent andthe balance substantially iron to form sheet or strip substantially freefrom. decarburization and surface grain growth effects.

10. In the production of stainless steel sheet or strip for use in thefabrication of pressed, bent,

stamped or deep-drawn articles, the art which includes, re-heating andthen hot-rolling ingots, blooms, slabs or billets comprising chromium 16per cent to 20 per cent, carbon .06 per cent to .30 per cent, nitrogengreater than .06 per cent but less than .20 per cent and the balancesubstantially iron to form. sheet or strip substantially free fromdecarburization and surface grain growth effects.

11. In manufactures of the class described, sound stainless steel sheetor strip free from decarburization and resultant coarse surface graintexture and minute checks and pits comprising chromium 10 per cent toper cent, nitrogen greater than .06 per cent but lessthan .20 per centand the balance substantially iron said sheet and strip being hot-rolledfrom an ingot, bloom, slab or billet.

12. In manufactures of the class described, highly polished cold-rolledstainless steel sheet or strip free from decarburization and resultantcoarse surface grain structure and pitting, comprising chromium 10 percent to 30 per cent, nitrogen over .06 per cent but under .20 per centand the balance principally iron, said sheet or strip being rolled fromhot-rolled sheet or strip.

13. In manufactures of the class described, highly polished, cold-drawnstainless steel rods or wire free from decarburization and resultantcoarse surface grain structure and pitting, comprising chromium 10 percent to 30 per cent,

ioned from sheet or strip of fine grain texture and tree fromdecarburiaatlon and surface grain growth eil'ects comprising chromium 10per cent to 30 per cent, nitrogen greater than .06 per cent but lessthan .20 per cent and the balance substantially iron.

16. In manufactures of the class described, highly polished articlesfashioned of re-heated and worked stainless steel tree fromdecarburization and surface grain growth eifects comprising chromium 10per cent to 30 per cent, nitrogen over .06 per cent but under .20 percent, and the balance iron.

17. In manufactures of the class described, highly polished pressed,bent, stamped, or deepdrawn stainless steel articles fashioned of sheetor strip of flnegrain texture free from decarburiaation and surfacegrain growth effects comprising chromium 10 per cent to 30 per cent,nitrogen greater than .07 per cent but less than .13 per cent and thebalance of the metal substantially iron, said sheet .or strip beingrolled from hot-rolled sheet or strip.

auapos 18. In the production of stainless steel plate, sheet or stripsubstantially free of surface defects, the art which includes, heatingor re-heating ingots, blooms, slabs or billets, comprising chromium 10per cent to 30 per cent, nitrogen in an amount less than .13 per centbut over .07 per cent, and the balance iron and then .hotrolling saidproducts to form plate, sheet or strip free from decarburization andsurface grain growth eifects by virtue of said nitrogen content.

19. In manufactures of the class described, sound stainless steel plate,sheet or strip free from decarburization and resultant coarse surfacegrain texture and minute checks and pits comprising chromium 10 per centto 30 per cent, nitrogen greater than .07 per cent but less than .13 percent and the balance substantially iron, said plate, sheet or stripbeing hot-rolled from an ingot, bloom, slab, or billet.

W1LLIAMB.ARNESS.

